This invention relates to paper processing wherein individual sheets are separated from a stack of sheets and further processed. This type of operation is performed by electrophotographic equipment, as well as a myriad of other paper processing equipment. The processing includes manipulating or handling the paper by the equipment in order to move the paper along a paper path. This invention can be used in conjunction with a wide variety of paper processing equipment, but was developed for use with electrophotographic equipment such as laser printers. The invention will be described in its application to electrophotography, it being understood that it also has applicability to other types of paper processing equipment.
Regardless of the particular configuration of the paper processing equipment, many have in common a paper pick, followed by a primary series of paper processing operations. The primary series of paper processing operations in the electrophotographic equipment, including the laser printer of the preferred embodiment is the developer and fuser stations. This includes the laser printer of the preferred embodiment of this invention.
In electrophotographic printing, individual sheets of paper are taken from a stack of paper in a paper supply and an image, corresponding to a pattern of electrostatic charges, is applied to the paper. The removal of an individual sheet of paper is referred to as a "pick."
In the Assignee's Laser Jet printers, such as the Assignee's Laser Jet II-P.TM. printer, a pick is accomplished by a roller having an asymmetric profile, called a "D roller," rotating past a fixed pad. The D roller has a higher coefficient of friction with paper than the fixed pad. The fixed pad is spring loaded against the D roller so that only a single sheet of paper normally is moved by the rotation of the D roller. This generally accomplishes the desired pick of a single sheet of paper which is thereby fed to the electrophotographic imager, and subsequently ejected as a printed page.
If this does not happen (i.e., a single sheet is not picked and successfully ejected after passing the imager), then sensors are used to detect a paper jam or related malfunction. The sensors generally consist of paper feed sensors, located before the imager, and forward sensors, located at the output of the imager, such as at the fuser station. This invention relates to an arrangement for paper feed sensors. The paper sensors typically sense the presence of paper in the paper supply and the presence of paper at the output of the D roller, and passage of the leading edge of the picked sheet. It should be noted that the forward sensors are located downstream of the paper feed sensors, and so are not forward of the paper feed sensors despite the terminology.
In the event of a sensed malfunction, a display on the printer indicates that the malfunction has occurred. The usual procedure requires the operator to investigate and remedy the malfunction, and press a "continue" or "operational reset" button. If the malfunction is a true paper jam, the equipment is partially opened, the wrinkled paper is removed from the paper path. Thereafter, the equipment is closed and the normal operation is continued upon pressing the button. On the other hand, the malfunction may be an empty paper supply. This of course does not require that paper be removed, but merely that the paper supply be replenished.
The reference to a paper out condition as a malfunction is not merely a problem of semantics, since the user may find it unnatural to see the equipment react to a paper out condition in the manner of a paper jam. In addition to the paper out condition being a function of the normal operation of the equipment, there are times that the paper out condition is even intentional. It is therefore important that the equipment be able to distinguish between a paper out condition and other paper supply malfunctions.
The detection of different paper positions can be accomplished by providing discrete sensors for each position. Under that arrangement, detection of presence of paper in the paper supply, the presence of paper at the output of the D roller, and passage of the leading edge of the picked sheet can be accomplished by at least three sensors. This enables the equipment to react to different types of malfunctions according to the malfunction. As an example, it is possible to discretely indicate a paper out condition, and even to automatically restart the equipment when the paper supply has been replenished.
Detection of different paper positions can also be accomplished without a sensor for detecting paper in the paper supply. In that case, the initial reaction of the printer to a print command is to initiate a pick and to later declare a paper jam.
For the purposes of detection of paper feed malfunctions, it is possible to ignore the forward sensors. Thus, malfunctions related to the fuser station are treated separately from malfunctions of the paper feed mechanism.
It is also desirable to reduce the number of sensing elements in order to reduce cost and complexity of the equipment. Thus, if a single switch or photosensor is used in place of two or three switches, the costs of switches and the chances of switch failure are reduced. The use of mechanical switches instead of photosensors has the advantage of lower cost, but also requires that force applied to operate the switches be present. As an alternative, photosensors are placed in the path of flags which are themselves mechanical arm and lever arrangements. This reduces the force applied to that necessary to cause the flag to move.
It is therefore desirable to provide an arrangement wherein different malfunctions are sensed and distinguished with a minimum of sensor apparatus. It is further desired to use a single photodetector for multiple sensing functions, with a minimum of mechanical complexity. It is further desired to reduce the number of sensors without substantially reducing the conditions sensed. In doing so, it is desired to utilize information which is obtained from other known factors.